1. Field of the Invention
The invention relates to a method for operating a magnetic-inductive flow meter, with a magnetic field generating apparatus for generating a magnetic field which permeates a flowing medium, and with a measuring apparatus for determining measured values which reproduce the field intensity of the electrical field which has been induced by the magnetic field in the flowing medium. The magnetic field generating apparatus comprises at least one electromagnet and one magnetic field conductor which has been penetrated at least partially by the magnetic field, the permeability of the magnetic field conductor being a nonlinear function of the magnetic field intensity.
2. Description of Related Art
Magnetic-inductive flow meters have been widely known in the prior art for decades. Reference is made by way of example to the literature citation Technical Flow Rate Measurement by Dr. Eng. K. W. Bonfig, 3rd edition, Vulkan-Verlag Essen, 2002, pp. 123 to 167 and also to the literature citation Principles of Magnetic-Inductive Flow Rate Measurement by Cert. Eng. Friedrich Hoffmann, 3rd ed., 2003, publication of the company KROHNE Messtechnik GmbH & Co. KG.
The basic principle of magnetic-inductive flow meters goes back to Michael Faraday who suggested the use of the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium as early as 1832. According to the Faraday Induction Law, in a flowing and electrically conductive medium which is permeated by a magnetic field an electrical field intensity arises perpendicular to the flow direction of the medium and perpendicular to the magnetic field.
The Faraday Induction Law is used in magnetic-inductive flow meters of the initially described type in that the magnetic field generating apparatus makes available a magnetic field which penetrates the flowing medium. Here the magnetic field is generated by the electromagnet and is guided by the magnetic field conductor. The magnetic field in the medium has at least one component which is perpendicular to the flow direction of the medium, as a result of which an electrical field intensity arises in the medium perpendicular both to the direction of the flowing medium and also to the direction of the magnetic field. The electrical field intensity is a measure of the flow rate of the medium through the magnetic-inductive flow meter and the measurement apparatus is made to determine the measured values which reproduce the field intensity.
The magnetic field conductor acquires its capacity to guide the magnetic flux which has been generated by the electromagnet by its permeability which is greater compared to the vicinity and which constitutes a lower resistance for the magnetic flux. The permeability is in no way constant here, but rather a nonlinear function of the magnetic field intensity.
Measured values which reproduce the induced electrical field intensity can be detected by at least two electrodes. The electrodes can be either in electrical contact with the medium or can be only capacitively coupled to the medium, and they are preferably located on a common axis which is aligned preferably parallel to the direction of the electrical field intensity. Due to this alignment, the measured values are maximum. Measured values can be, for example, measured voltage values which upon electrical contact of the measuring electrodes with the medium are directly measured or in capacitive coupling of the measuring electrodes to the medium result from the displacement current.
Magnetic-inductive flow meters are usually operated with alternating magnetic fields. An alternating magnetic field causes oscillating measured values, as a result of which the at least partial compensation of noise, such as electrochemical noise voltages, is possible whose change over time is slower than the change of the alternating magnetic field over time.
The alternating magnetic field can be a harmonic alternating magnetic field. In harmonic alternating magnetic fields the change of the magnetic field intensity over time is a harmonic oscillation. A harmonic alternating magnetic field can be produced by feeding the electromagnet from an existing AC voltage network. The operation of magnetic-inductive flow meters with a harmonic alternating magnetic field however has disadvantages, such as can be taken for example, from DE 199 07 864 A1, column 1, line 53 to column 2, line 13, and corresponding U.S. Pat. No. 6,453,754 B1.
The disadvantages which arise in the operation of a magnetic-inductive flow meter with a harmonic alternating magnetic field can be avoided by operating with an alternating magnetic field which is a switched constant magnetic field. A switched constant magnetic field consists of the periodically repeating sequence of at least two intervals, in each of the intervals the magnetic field being constant after a transient reaction, and the magnetic fields being different in two successive intervals. Different magnetic fields are produced by energizing the electromagnet with different current values. A current value is characterized by the amount of current and the direction of the current. Thus the magnetic fields can differ by different magnetic field intensities and/or different magnetic field directions. Generally a switched constant magnetic field consists of two intervals of the same length and the magnetic fields of the intervals transiently have the same magnetic field intensity, but opposite magnetic field directions.
The measured values which reproduce the field intensity of the electrical field which has been induced in the flowing medium by the magnetic field are proportional to the magnet flux density. If in addition to the magnetic field which has been generated by the electromagnet, there is also a parasitic magnetic field which contributes to the magnetic flux density in the medium, the flow rate measurement is adulterated. The parasitic magnetic field can be produced, for example, by other magnetic-inductive flow meters or electric motors in the vicinity of the magnetic-inductive flow meter, and therefore, can travel into the medium via the magnetic field conductor.